1. Field of the Invention
The present invention relates generally to surface-treated steel sheets for use in cans such as xe2x80x9cdrawn and ironedxe2x80x9d (DI) cans, food cans, beverage cans, and the like. More particularly, it relates to a tin-plated steel sheet having excellent overcoat adhesion property and superior resistance to discoloration and rust.
2. Description of the Related Art
Tin-plated steel sheets are widely used as surface-treated steel sheets for use in cans. The tin-plated steel sheets are usually produced by first plating a cold-rolled steel sheet with tin and then immersing or electrolyzing the resulting plated steel sheet in an aqueous solution of hexavalent chromium compounds such as chromates or dichromates. Through such immersion or electrolysis, which is known as a chromating process, chromium oxides are formed on the plated tin layer to provide a chromate coating. The chromate coating, which prevents growth of tin oxides, suppresses xe2x80x9cyellowingxe2x80x9d, i.e., discoloration of the tin-plated steel sheet surface to a yellowish color (hereinafter also referred to as discoloration resistance) and enhances overcoat adhesion property and resistance to rust.
However, chemical conversion treatment using an aqueous solution of hexavalent chromium compounds such as chromates or dichromates requires a significantly high cost for securing the safety of the work environment and for effluent treatment. Moreover, leakage of the liquid used in the chromating process, if caused by an accident or the like, inflicts significant damage upon the ambient environment. The recent trend toward environmental protection has promoted regulations on the use of chromium; thus, there is an increasing need for chromium-free chemical conversion treatment for the surface-treated steel sheets for use in cans having improved resistance to discoloration and rust and overcoat adhesion property.
Examples of chromium-free chemical conversion treatments for surface-treated steel sheets for use in cans which replace conventional chromating processes are as follows. Japanese Unexamined Patent Application Publication No. 55-24516 discloses a method for forming chromium-free chemical conversion coating on a tin-plated steel sheet, the method comprising direct-current electrolysis of the tin-plated steel sheet in a phosphate-system aqueous solution using the tin-plated sheet as a cathode. Japanese Unexamined Patent Application Publication No. 1-32308 discloses a chromium-free electrolytic tin-plated steel sheet for use in seamless cans, comprising a chemical conversion coating formed on a tin plating layer, the chemical conversion coating including either phosphorus (P) alone or phosphorus (P) and aluminum (Al).
However, all of the chemical conversion coatings disclosed in the above-described publications are hardly comparable to the conventional chromate coating formed using dichromic acid or chromic acid when their comprehensive performance is evaluated in terms of overcoat adhesion property and resistance to discoloration and rust.
Accordingly, it is an object of the present invention to provide a tin-plated steel sheet having excellent overcoat adhesion property and resistance to discoloration and rust without having to contain, in its chemical conversion coating, chromium, which is harmful to the environment.
The present invention provides a tin-plated steel sheet comprising: a base steel sheet; a tin plating layer coating approximately more than 97.0% of the base steel sheet; and a chemical conversion coating having approximately 0.5 to 100 mg/m2 phosphorus and approximately 0.1 to 250 mg/m2 silicon formed on the tin plating layer and an unplated region corresponding to approximately less than 3.0%.
Preferably, silicon contained in the chemical conversion coating is derived from a silane coupling agent. More preferably, the silane coupling agent contains an epoxy group.
The tin-plated steel sheet may further comprise an alloy layer disposed on the base steel sheet and at least beneath the tin plating layer.
Preferably, the alloy layer comprises at least one layer selected from the group consisting of a Fexe2x80x94Sn alloy layer, a Fexe2x80x94Ni alloy layer, a Snxe2x80x94Ni alloy layer, and a Fexe2x80x94Snxe2x80x94Ni alloy layer. More preferably, the alloy layer comprises a composite alloy layer comprising a Fexe2x80x94Ni alloy layer having a mass ratio Ni/(Fe+Ni) in the range of approximately 0.02 to 0.50 and a Fexe2x80x94Snxe2x80x94Ni alloy layer disposed on the Fexe2x80x94Ni alloy layer.
Preferably, the total Sn content of the tin plating layer and the alloy layer is in the range of approximately 0.4 to 6.0 g/m2.
The present invention will now be described in detail.
Chromium-free chemical conversion coatings formed on tin plating layers by known methods rarely achieve all of the required overcoat adhesion property and resistance to discoloration and rust, which are the key properties of steel sheets for use in cans.
The present inventors have conducted extensive research to overcome the above problem of tin-plated steel sheets and found that all of the above required properties can be fulfilled by forming a chemical conversion coat containing phosphorus (P) and silicon (Si) on a tin plating layer.
In particular, a chemical conversion solution containing P and a silane coupling agent is used to form a chemical conversion coating containing adequate amounts of P and Si on the tin plating layer. Alignment of the functional groups contained in the silane coupling agent enhances the adhesion property to the overcoat for inner surfaces of cans. That is, the chemical conversion coating improves compatibility and reactivity to the overcoat and thereby yields superior overcoat adhesion property. Moreover, the chemical conversion coating functions as a protective coating to improve resistance to discoloration and rust.
The detailed configuration of the present invention will now be described.
In the tin-plated steel sheet of the present invention, the base steel plate needs to have at least one surface satisfying the requirements of the present invention. No limit is imposed as to the type of the base steel sheet; a cold-rolled steel sheet is generally employed.
The present invention can be applied to a tin-plated steel sheet. The tin-plated steel sheet may be formed by directly plating a base steel sheet with tin or by forming an alloy layer on the base steel sheet and then plating the alloy layer with tin. For example, a tin-plated steel sheet according to an embodiment of the present invention has a tin plating layer directly formed on almost the whole surface of a base steel sheet with a coating coverage exceeding 97%. Another embodiment of a tin-plated steel sheet has an alloy layer between the tin plating layer and the base steel sheet. In this embodiment also, the coating coverage by the tin plating layer exceeds 97%; accordingly, an unplated portion may remain at less than 3.0%. The unplated portion may be the base steel sheet or the alloy layer. In the present invention, the term xe2x80x9ccoating coveragexe2x80x9d refers to the percentage of the surface of the material to be plated covered by the tin plating layer. In this invention, a sufficient resistance to rust can be obtained with a coating coverage, i.e., the percentage of the base steel sheet and/or the alloy layer covered by the tin plating layer, exceeding 97%.
As described above, the present invention includes an embodiment in which an alloy layer is provided on the base steel sheet and at least beneath the tin plating layer. The alloy layer preferably includes at least one selected from a Fexe2x80x94Sn alloy layer, a Fexe2x80x94Ni alloy layer, a Snxe2x80x94Ni alloy layer, and a Fexe2x80x94Snxe2x80x94Ni alloy layer. More preferably, the alloy layer is a composite alloy layer comprising a Fexe2x80x94Ni alloy layer having a Ni/(Fe+Ni) mass ratio of approximately 0.02 to 0.50 and a Fexe2x80x94Snxe2x80x94Ni alloy layer on the Fexe2x80x94Ni alloy layer. This alloy layer, which has also been employed in the conventional tin-plated steel sheets, improves resistance to corrosion and rust. Since the hardness of the alloy layer is high compared to that of the tin plating layer, the alloy layer degrades the workability. When the tin-plated steel sheet of the present invention is applied to DI cans requiring high workability, the tin plating layer is preferably formed directly on the base steel sheet without the alloy layer.
Next, a specific method for making the alloy layer is explained.
Generally, in making a Fexe2x80x94Sn alloy layer, tin plating is first directly performed on a base steel sheet and then heating is performed to melt Sn. This heating is called a reflow process and is a simple, easy process for forming the Fexe2x80x94Sn alloy layer.
In making a Fexe2x80x94Snxe2x80x94Ni alloy layer, a common preliminary process of Ni plating such as Ni flash plating or Ni diffusion is performed on a base steel sheet. Tin plating is then performed followed by a reflow process to melt the plated tin by heating so as to make the Fexe2x80x94Snxe2x80x94Ni alloy layer. When Ni flash plating is performed in making the alloy layer, the Ni coating weight is preferably in the range of approximately 0.005 to 0.05 g/m2. At a coating weight of 0.005 g/m2 or more, sufficient corrosion resistance can be obtained. At a coating weight of 0.05 g/m2 or less, the dissolving rate of Sn under a corrosive environment can be decreased and sufficient rust resistance can be obtained.
In making a Nixe2x80x94Sn alloy layer, Ni flash plating and then tin plating are performed. In this manner, Ni and Sn are alloyed at normal temperatures without a reflow process, and the Nixe2x80x94Sn alloy layer can be easily formed. In this case also, sufficient rust resistance can be achieved by controlling the amount of Ni coating within the above-described range.
In making a composite alloy layer comprising a Fexe2x80x94Ni alloy layer and a Fexe2x80x94Snxe2x80x94Ni alloy layer on the Fexe2x80x94Ni alloy layer, a base steel sheet is first plated with Ni, and annealing is performed in a 10 vol. % H2+90 vol. % N2 atmosphere at approximately 700xc2x0 C. in order to diffuse Ni and to form the Fexe2x80x94Ni alloy layer. Next, the Fexe2x80x94Ni alloy layer is plated with tin and is heated at a temperature above the melting point of Sn to form the Fexe2x80x94Snxe2x80x94Ni alloy layer, thereby forming the composite alloy layer. Note that in making the composite alloy layer, the mass ratio Ni/(Fe+Ni) in the Fexe2x80x94Ni alloy layer is preferably in the range of approximately 0.02 to 0.50. At a mass ratio of Ni/(Fe+Ni) of approximately 0.02 or more, sufficient corrosion resistance can be obtained. At a mass ratio of Ni/(Fe+Ni) of approximately 0.50 or less, the dissolving rate of Sn under a corrosive environment can be decreased and improved resistance to rust can be obtained. Moreover, the Fexe2x80x94Ni alloy layer alone can exhibit improved corrosion resistance when the Fexe2x80x94Ni layer has Ni diffused therein at a mass ratio of Ni/(Fe+Ni) in the range of approximately 0.02 to 0.50. The mass ratio of Ni/(Fe+Ni) can be obtained by analyzing Fe and Ni in the depth direction using micro auger electron spectroscopy (xcexc-AES), integrating the product of each relative sensitivity coefficient and each peak value with respect to the depth, and calculating using the following formula: the integrated value of Ni/(the integrated value of Ni+the integrated value of Fe).
In the present invention, the total coating weight of tin contained in the tin plating layer and the alloy layer is preferably in the range of approximately 0.4 to 6.0 g/m2. This is because a Sn coating weight of approximately 0.4 g/m2 or more is enough to obtain sufficient resistance to rust. At a Sn coating weight exceeding approximately 6.0 g/m2, however, the cost becomes high although the performance is satisfactory. More specifically, the term xe2x80x9cthe total coating weight of tinxe2x80x9d refers to the amount of tin contained in the tin plating layer when no tin is contained in the alloy or when no alloy layer is provided. When Sn is contained in the alloy layer, the term refers to the amount of Sn contained in the tin plating layer and the amount of Sn contained in the alloy layer in total. The Sn coating weight can be measured by coulometric analysis or surface analysis using fluorescent X-rays.
Another important feature of the present invention is to provide a chemical conversion coating containing approximately 0.5 to 100 mg/m2 of phosphorus (P) and approximately 0.1 to 250 mg/m2 of silicon (Si) on the tin plating layer coating approximately more than 97.0% and on the unplated portion not covered by the tin plating layer which is approximately less than 3.0%. The unplated portion is either base steel sheet or the alloy layer. In the present invention, this coating is provided on the tin plating layer as well as the unplated portion. This coating, also referred to as xe2x80x9cchemical conversion coatingxe2x80x9d in this specification, is preferably formed using a chemical conversion solution containing phosphorus and silane coupling agent.
The P content in the coating must be in the range of approximately 0.5 to 100 mg/m2. At a content of 0.5 mg/m2 or more, sufficient overcoat adhesion property and resistance to discoloration can be achieved. The upper limit is approximately 100 mg/m2 because defective coating can be prevented and sufficient overcoat adhesion property and workability can be obtained. The P content can be measured by surface analysis using fluorescent X-rays, for example.
The chemical conversion coating containing P is preferably formed by a phosphate-system chemical conversion. The chemical conversion solution preferably contains free phosphoric acid, a metal phosphate such as sodium phosphate, aluminum phosphate, potassium phosphate or the like, and/or monohydrogenphosphate as the supply source of phosphorus at an amount of approximately 1 to 80 g/l in terms of phosphate ions. The chemical conversion solution may further contain a salt including Sn, Fe, or, Ni, such as SnCl2, FeCl2, NiCl2, SnSO4, FeSO4, NiSO4, or the like. In such a case, an oxidizing agent such as sodium chlorate, nitrite, or the like and an etchant such as fluorine ions may be added as an accelerator, if necessary. The chemical conversion coating containing phosphorus can be formed by immersion or electrolysis of the tin-plated steel sheet using a phosphate-system chemical conversion solution.
The Si content in the chemical conversion coating must be in the range of approximately 0.1 to 250 mg/m2. Si contained in the coating is preferably introduced from the silane coupling agent contained in the chemical conversion solution. A typical chemical conversion solution can be expressed as RSi(xe2x80x94X) (xe2x80x94ORxe2x80x2)2 or as XSi(xe2x80x94ORxe2x80x3)3, wherein R, Rxe2x80x2 and Rxe2x80x3 are alkyls of the same or different types and X is a monovalent substituent.
The silane coupling agent forms a silanol group (xe2x89xa1Sixe2x80x94OH) by hydrolysis of the alkoxysilyl group (xe2x89xa1Sixe2x80x94ORxe2x80x2) and adheres onto the metal surface by a condensation reaction with a hydroxyl group (xe2x80x94OH) present on the metal surface. The substituent X in the above formula readily aligns with the overcoat or resin disposed thereon so as to be compatible with or bonded to these overcoatings.
Examples of the silane coupling agent are 3-methacryloxypropyltrimethoxysilane, 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, 3-glycidoxypropyltrimethoxysilane, N-2-(aminoethyl)-3-aminopropyltrimethoxysilane, N-2-(aminoethyl)-3-aminopropylmethyldimethoxysilane, 3-aminopropyltriethoxysilane, N-phenyl-3-aminopropyltrimethoxysilane, 3-mercaptopropylmethoxysilane, 3-chloropropyltrimethoxysilane, vinyltriethoxysilane, vinyltris (2-methoxyethoxy) silane, N-2-(aminoethyl)-3-aminopropyltrimethoxysilane, N-2-(aminoethyl)-3-aminopropylmethyldimethoxysilane, and 3-aminopropyltriethoxysilane. A silane coupling agent having a substituent X including an epoxy group is especially preferable. Examples of such silane coupling agents are 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane and 3-glycidoxypropyltrimethoxysilane. This is because they have excellent compatibility and reactivity to the epoxy-system overcoat used to coat the inner surfaces of the can.
In the present invention, the Si content in the chemical conversion coating is in the range of approximately 0.1 to 250 mg/m2 because the overcoat adhesion property can be significantly improved thereby. Sufficient overcoat adhesion property can be obtained at a Si content of approximately 0.1 mg/m2 or more. The upper limit is approximately 250 mg/m2 because self condensation of the unreacted moiety of the silane coupling agent can be prevented without degrading the overcoat adhesion property. The Si content can be measured by surface analysis using fluorescent X-rays.
To form the coating containing P and Si, a chemical conversion coat containing P is first formed using the above-described phosphate-system chemical conversion solution and then treating the resulting coating in a solution of a silane coupling agent diluted with water. Note that when the treatment is performed using the solution of the silane coupling agent diluted with water, repelling may occur due to the poor wettability of the surface. The repelling can be prevented using a solution containing alcohol. For example, a solution containing approximately 50 mass % or more of ethanol, approximately 0.5 to 20 mass % of silane coupling agent, and the balance being water can be used to achieve uniform treatment. The treatment using the solution containing the silane coupling agent can be performed by application and drying or by immersion. When the silane coupling agent is added to the above-described phosphate-system chemical conversion solution for forming a coat containing P, a chemical conversion coat containing P and Si can be formed using only one solution. In this case, the pH of the chemical conversion solution is controlled within the range of approximately 1.5 to 5.5 so as to homogeneously dissolve the silane coupling agent in the chemical conversion solution and to achieve excellent overcoat adhesion property. The mass ratio Si/P in the chemical conversion coat is preferably in the range of approximately 0.05 to 100, since the overcoat adhesion property and the corrosion resistance after application of the overcoat can be remarkably improved.
As described above, the present invention fulfills all the requirements of excellent overcoat adhesion property and superior resistance to discoloration and rust by providing a coat containing P and Si in the above-described amount on a tin plating layer formed on a surface of the steel sheet.
Next, an exemplary method for making the tin plated steel sheet of the present invention will be described. For the purpose of the explanation, the alloy layer is formed as a composite alloy layer having an Fexe2x80x94Ni alloy layer and an Fexe2x80x94Snxe2x80x94Ni alloy layer on the Fexe2x80x94Ni alloy layer.
As described above, the Fexe2x80x94Ni alloy layer is first formed by diffusion of Ni in the base steel sheet. Next, tin plating is performed thereon, and, subsequently, reflow treatment is performed at a temperature above the melting point of tin (231.9xc2x0 C.) to form a composite alloy layer having an Fexe2x80x94Snxe2x80x94Ni alloy layer on the Fexe2x80x94Ni alloy layer. Next, a chemical conversion treatment is performed by immersing the tin-plated steel sheet having the composite alloy layer thereon into a chemical conversion solution. Note that in the present invention, in order to remove tin oxides formed on the surface after the reflow treatment, cathodic treatment may be performed at approximately 1 C/dm2 in an approximately 15 g/l sodium carbonate aqueous solution.
The chemical conversion solution is prepared by adding approximately 0.5 to 20.0 mass % of a silane coupling agent to an aqueous solution containing approximately 1 to 80 g/l of phosphoric acid based on phosphate ions, approximately 0.001 to 10 g/l of stannous chloride based on stannous ions, and approximately 0.1 to 1.0 g/l of sodium chlorate. The temperature of the chemical conversion is preferably approximately 40 to 60xc2x0 C., and the immersion time is preferably approximately 1 to 5 seconds. In this particular example, the chemical conversion is performed at a temperature of 50xc2x0 C. for an immersion time of 5 seconds. The tin-plated steel sheet after the chemical conversion is dried by hot air of approximately 35 to 150xc2x0 C.
Another method for forming the chemical conversion coat includes treating the tin-plated steel sheet with a chemical conversion solution not containing the silane coupling agent, uniformly applying the silane coupling solution on the resulting tin-plated steel sheet so as to form a silane coupling layer, and drying the resulting sheet by heating the steel sheet to a surface temperature of approximately 50 to 150xc2x0 C. In such a case, a silane coupling solution containing, for example, approximately 50 mass % or more of ethanol approximately 0.5 to 20 mass % of the silane coupling agent, and the balance being water, can be used.